The development of devices used to monitor intrauterine pressure in labor and delivery has vastly improved the ability of a physician to assess the health of both fetus and mother during childbirth. The use of such a device, often in conjunction with a fetal heart monitor, serves to warn a physician of potential problems before they become life-threatening. For example, an intrauterine pressure monitor allows a physician to assess whether adequate oxygen and nutrients are being transferred from mother to fetus and whether the rate, duration, and intensity of the intrauterine contractions are acceptable.
Two types of devices are commonly used to monitor intrauterine pressure in a mother during labor and delivery. Often an external device secured to the mother's abdomen is used. For example, a pressure sensitive button, called a tocotransducer, capable of measuring the frequency and duration of intrauterine contractions is available. This type of external device has the advantage of being completely noninvasive. Therefore, it eliminates any threat of infection and risks little discomfort to the mother.
External devices frequently provide inaccurate measurements of intrauterine pressure, however, because they are sensitive to excessive movement and to obesity of the mother. In addition, external devices are poorly capable of measuring the actual intensity of intrauterine contractions. For these reasons, an external device is ineffective in many childbirth situations, specifically, when there is risk of complication and the physician desires very accurate information.
Devices capable of measuring intrauterine pressure when inserted into the uterus provide a more accurate measure of events during labor than the external devices and, in particular, a more accurate measure of intrauterine contraction intensity. Because the nature and duration of the contraction is measured directly, the intrauterine devices minimize the potential for inaccurate readings due to the mother's movement or obesity.
Historically, the most common intrauterine devices are those which use a liquid-filled catheter which, after it is inserted into the uterus of the mother, is hydraulically connected to an external pressure transducer. The pressure transducer is typically mounted to a bedside IV stand and is connected to a monitor device. The monitor may be a cathode ray tube display, a digital display and recording device, a printer, or a plotter.
Typically, the catheter is primed with a sterile solution to provide a continuous column of liquid from the pressure transducer to the tip of the catheter within the uterus. Upon contraction of the uterus, the increased intrauterine pressure causes displacement of the liquid in the catheter which is subsequently measured by the pressure sensor and transmitted to the monitor. The monitor displays the mother's intrauterine pressure as a function of time.
The pressure transducer may be used with a disposable dome which fits over the transducer diaphragm. Typically, there are two ports in the dome, a side port which is connected to the liquid-filled catheter after it is primed with a sterile solution, and a second port which is used for calibration and zero balancing.
There are two significant disadvantages of this type of intrauterine device. First, cumbersome manipulations and time are required to fill and prime the catheter with sterile solution. Second, air bubbles frequently enter the open end of the catheter. Such air bubbles require flushing of the catheter with a sterile solution or replacement of the entire catheter, either of which may cause discomfort to the mother.
An improved, in-dwelling, catheter apparatus useful for monitoring changes in intrauterine pressure during labor is described in U.S. Pat. No. 4,966,161 issued to Wallace et al. The catheter has apertures near its distal end, placing a first chamber formed in the distal catheter tip in fluid communication with the fluid of the uterus. Liquids enter the first chamber and form a liquid column with a liquid-air interface. A second chamber, filled with air, is defined by an interior lumen extending the length of the catheter up to the first chamber. An air column is formed by air trapped in the second chamber. The relative volumetric capacity of the first and second chamber is such that, under maximum uterine pressures, the trapped air column will limit the liquid column in the first chamber, minimize hydrostatic pressure error, and prevent fluid from entering the second chamber. Uterine pressure is transmitted from the partially liquid-filled first chamber, through the air-filled column of the second chamber, to the diaphragm of the pressure transducer--which is also part of the catheter.
The design of the '161 device eliminates the requirement for the cumbersome flushing and filling of the catheter found in previous designs. That device requires, however, that the volumes of the two chambers be carefully controlled. Moreover, the pressure sensor is mounted in the catheter and is in direct contact with the intrauterine fluid. Therefore, the entire catheter and pressure sensor device must be discarded after use to prevent transmitting infectious agents from one patient to another. This presents a significant economic disadvantage; the pressure sensor is one of the most costly components of the device.
U.S. Pat. No. 5,000,049 issued to Cooper et al. discloses a diaphragm-type pressure gauge for use with medical apparatus such as catheters. The gauge has a pressure diaphragm which responds to a change in fluid pressure and activates a mechanical pressure signal translation mechanism. A second, sealing diaphragm may be disposed adjacent the first diaphragm to transmit pressure changes thereto without allowing the passage of materials between the two diaphragms. (Alternatively, the pressure diaphragm may be sealed with a biocompatible material.) The fluid may be provided by a catheter whose wall is disposed against the pressure diaphragm so that the diaphragm senses fluid pressure changes without contact between the diaphragm and the fluid. The gauge is essentially reusable because the pressure diaphragm does not contact the fluid.
The design of the '049 device incorporates a relatively complex method of connecting the gauge and transducer to the measuring circuit, involving, in the preferred embodiment, two diaphragms (one in the wall of a tube). Moreover, the design does not focus on the catheter and, consequently, fails to provide a lumen in the catheter for adding fluid to and removing fluid from the body. Finally, the '049 design risks significant attenuation of the pressure signal because it uses a mechanical pressure signal translation mechanism and the catheter wall to transmit the pressure signal.
U.S. Pat. No. 4,944,307 issued to Hon et al. discloses an intrauterine catheter which is connected at one end, through an adapter, to the input of a pressure transducer and closed at its uterine end by a flexible membrane. The catheter is filled with a sterile fluid. No air is in either the catheter or pressure transducer. A change in intrauterine pressure is transmitted to the pressure transducer without contact between the intrauterine fluid and the pressure transducer.
Although the pressure measuring system of the '307 patent can be zeroed while the catheter is in the uterus, the procedure required is relatively complex. Also relatively complex is the procedure required to set up and insert the system in the uterus. The catheter may be filled with a silicone-based fluid, which could enter the uterus upon development of a leak in the tip membrane. Finally, the design of the '307 device fails to provide a lumen in the catheter for adding fluid to and removing fluid from the body.
U.S. Pat. No. 5,105,820 issue to Moriuchi et al. discloses a disposable pressure transducer for measuring fluid pressure which includes a sensor assembly in a housing. The housing has a fluid chamber and a chamber accomodating the sensor assembly; the two chambers are integrally formed. Pressure changes are transferred to the fluid chamber and then to a diaphragm of a pressure sensor through a pressure-transmitting medium disposed between the two chambers. The pressure-transmitting medium thus isolates the sensor from the fluid whose pressure is being measured.
The apparatus of the '820 reference is costly because the sensor is disposable. Like the pressure measuring system of the '307 patent, there is no structure disclosed in the disposable pressure transducer apparatus of the '820 patent to zero the apparatus instantaneously. Finally, the design of the '820 device fails to provide a lumen in the catheter for adding fluid to and removing fluid from the body.
To overcome the shortcomings of the prior art, a new intrauterine pressure catheter system is provided. The general object of the present invention is to provide a system capable of monitoring intrauterine pressure in a woman during labor and delivery which is economical to use and reuse. In order to achieve that general object, a more specific object is to provide an intrauterine pressure catheter system in which all of the components which come into contact with the mother's bodily fluids and tissues are sterilizable, inexpensive, and disposable. Related objects are to separate the expensive pressure sensor from the disposable part of the system and to isolate the pressure sensor from the mother so that it can be reused without risk of transferring infectious agents from one patient to another.
Further objects are to provide a system which is safe to both the mother and the physician. Its use must create minimal discomfort to the mother and provide accurate information to the physician. In addition, the use of the system must avoid the complexity of running a liquid-filled tube from the catheter to a stand-mounted transducer.
Still another object of the present invention is to provide a system in which the transducer can be zeroed after the catheter has been inserted into the uterus and the appropriate monitoring device connected to the transducer. Yet another object is to provide an intrauterine pressure catheter system which allows fluid samples to be withdrawn from, and fluids to be infused into, the uterus.